What makes Snapchat popular is their use of pictures. Turning the thesis of your point into a totally public, 160 character or less, textual format is effort. A lot of effort.

Pictures are what they are. They're also extremely difficult to parse (no practical reason to retain your message, deletion is a cost effective decision), and they're the easiest thing in the world to make. Who wants to carefully word your tweets when your camera will do?